It sounds to me like there's been some confusion in this thread between "scaling plans" as in "setting Autocad viewport and paperspace scales to appropriate values" vs. "measuring printed plans with a scale".
We can setup the viewport and paperspace scales EXACTLY. There's no reason to need an "approximation" there. So the only issue then is how accurately the printer can print. These days, in my experience, the vast majority of printers introduce negligible distortion, and you can use a scale on those plans just fine. Scaling off of a copy of a drawing tends to be worse, though, as I still see most scanners that distort the scanned image in one direction more than the other.
But we also get into another issue, that of "accuracy" vs. "precision". In this thread, the word "accuracy" has been repeatedly used when people are really talking about "precision". We can print out a very accurate plan set, but we are limited in how precisely we can measure that line by putting an engineer's scale up against that paper print.
